(a) Field of the Invention
Steel is the most important metallic material, and the demand for steel has increased remarkably in recent years. It has been possible to meet this increasing demand for steel only by rapid progress and development in pig iron-manufacturing and steel-manufacturing techniques. One area of progress has been finding solutions to the problem of improving the quality of metallurgical coke to be used for blast furnaces and of developing methods for the mass-production of such coke.
Coke for use in blast furnaces is required to have the following properties:
(1) The strength expressed as the 15 mm index in the drum test (wherein the coke is rotated 30 times in a drum under conditions specified in JIS K2151 and the 15 mm index DI.sub.15.sup.30, namely the proportion (%) of coke particles having a size not smaller than 15 mm after the test, is determined) must be 90 to 95.
(2) The particle size of coke must be relatively uniform in the range of from about 25 to about 75 mm.
(3) The ash content must be as low as about 10 to about 11%.
(4) The sulfur content muxt be low, not exceeding 0.8%.
(5) The porosity must be in the range of from 40 to 60%.
(6) The reactivity index as determined according to JIS K2151 must be in the range of from 15 to 25.
By using coke meeting the foregoing requirements, the blast furnace operation can be performed stably at high efficiency.
Coke meeting the above requirements has heretofore been prepared from a mixture of hard coking coal and soft coking coal mainly because use of these feed coals gives the strength necessary for stable blast furnace operation. Although hard coking coal has been indispensable for production of blast furnace coke, the world's supply of hard coking coal is low and coke makers are finding it difficult to secure a necessary quantity of hard coking coal.
It is therefore anticipated that with increase of iron and steel production, it will be even more difficult to secure the necessary quantity of hard coking coal and the price thereof will inevitably increase rapidly.
This problem is very serious in the field of steel manufacture and some solution is strongly desired. Namely, development of a process for production of blast furnace coke in which hard coking coal need not be used at all or in which the amount of hard coking coal used is drastically reduced is desired in the art. Coke prepared according to the present invention meets this need.
Another problem to be solved in the production of coke is the problem of environmental pollution. It is often said that the main cause of environmental pollution in the iron industry is the coking plant, and various complaints are heard concerning the chamber-type coking oven. All the operations from charging of feed coal to withdrawal of produced coke are performed in the open state and sufficient measures are not being taken to prevent discharge of coal particles, dusts, gases, tar and nitrogen oxides. Of course, adoption of a closed system has heretofore been attempted in the art, but none of the attempts has substantially overcome the problems of the existing coking oven of the chamber type. Moreover, when the coking operation is carried out in the chamber-type coking oven, various steps must be conducted manually and the process cannot be worked in a continuous manner. Therefore, it is impossible to perform the coking process automatically while attaining a labor-saving effect.
When the carbonizer of the present invention is used, all the steps can be performed in a closed system and automation of the coking process becomes possible. In short, the present invention successfully overcomes all the defects and disadvantages involved in known carbonizers of the chamber type.
(b) Description of the Prior Art
Known carbonizers are roughly divided into two types: namely the type in which by-products are not recovered and the type in which by-products are recovered. The beehive coke oven can be mentioned as a typical instance of the former type, and typical instances of the latter type include (1) a horizontal flue type, e.g., the Solvay furnace and (2) a vertical flue type, e.g., the Koppers and Otto furnaces. Each of these known carbonizers involves various defects such as pointed out in (a) above.
Although formed coke has not yet been manufactured on an industrial scale, it can be obtained by the following steps.
Non-coking coal pulverized to have a prescribed particle size is incorporated into feed coal at an appropriate ratio. This mixing ratio is determined on the basis of such factors of the feed coal as ash content, volatile content, sulfur content, coking property fluidity and swelling property. Then, a binder such as pitch or bitumen is added to the resulting coal blend and the blend is kneaded at a temperature sufficient to melt the binder. The kneaded blend is briquetted under compression to obtain briquette coal. This briquette coal is charged in a high temperature carbonizer to effect coking and obtain formed coal.
The process for preparing formed coke can be roughly divided into two steps, namely the briquetting step and the carbonizing step.
The molding step involves various problems still unsolved, but if the feed coal and briquetting method are appropriately chosen, mass production is possible to some extent. A suitable oven or furnace for performing the latter carbonizing step has not yet been developed.
Shaft furnaces, travelling grate furnaces, rotary kilns or chamber furnace type coking ovens have heretofore been used as carbonizers for performing the carbonizing step, and among them, shaft furnaces are most promising and test plants have already been constructed. When a furnace of this type is employed, a product is prepared while briquette coal is allowed to fall in the furnace by its own weight, and the process seems advantageous. However, although briquette coal falls smoothly in case of a small-scale pilot plant, in case of a large plant, it is difficult to achieve uniform heating so that agglomeration of briquette coal particles (a kind of the sintering phenomenon) results. Consequently, such troubles as hanging occur and a product having a good quality cannot be obtained.
Travelling grate furnaces are now used in plants for sintering and pelletizing iron ores, and some good results have been attained. However, when a furnace of this type is employed, a perfect seal cannot be attained. More specifically, since a belt conveyor disposed in the furnace includes parts moving upwardly and downwardly (at both the ends), no complete seal can be attained at these moving parts. When the furnace is used for sintering or pelletizing iron ore according to the known techniques, the operation is carried out while sucking air and therefore no particular disadvantage is brought about even if complete seal is not attained. However, this fact results in a fatal defect when the furnace is used for carbonizing coal. More specifically, generation of poisonous gases cannot be obviated in carbonization of coals so that environmental pollution occurs. Therefore, when a travelling grate furnace is employed, the entire plant must be contained in a closed chamber.
Rotary kilns cannot be adopted because formed coal or coke is broken by rotation.
Since chamber furnace-type coking ovens heretofore used are of the external heating type, the width is very narrow but the length is long (width = about 50 to about 40 cm, length = about 15 m, height = about 5 to about 7 m). When such an oven is used, although carbonization can be performed without any particular problems, it is difficult to withdrawn the carbonization product, i.e., coke. More specifically, although discharge can be accomplished conveniently by extrusion in case of coke having a high strength (DI.sub.15.sup.30 value of at least 85) which is prepared from feed coal containing hard coking coal, as is well known in the art, in case of coke insufficient in the strength prepared from non-coking coal or weakly coking coal, discharge of the product cannot be performed conveniently by extrusion or the like because of clogging.
In order for coke to be discharged smoothly from a narrow and long furnace such as the chamber furnace type coking oven, it is necessary for the coke to have a sufficient strength as pointed out above and in addition, the coke should form one rigid body. When the coke forms one rigid body, if a small force is applied at one end of the furnace, the coke can easily be discharged from the other end. However, if the coke is present not in the form of one rigid body but in the disintegrated fragmentary state, even when a force is applied at one end of the furnace, the effect is only to close up voids or clearances and the coke cannot be discharged or withdrawn. If a larger force is applied, the refractory bricks of the furnace will be broken. In case of formed coke, the strength is sufficient to perform withdrawal by extrusion but respective particles are present independently and they do not form one rigid body. Therefore, the formed coke cannot be discharged by extrusion because of clogging.
In addition, there can be mentioned a bottom open hearth-type furnace in which the product is withdrawn by opening the furnace hearth. However, a large furnace of this type cannot be expected to be practical. The maximum capacity among existing furnaces of this type is about 3 tons.
As will be apparent from the foregoing illustration, there has not yet been developed a coking furnace capable of mass production of formed coke.
As a result of various experiments and investigations, we have now succeeded in developing a horizontal circulating carbonizer which is quite different from known carbonizers in structure and function and a process for the preparation of formed coke using this novel carbonizer.